[flang] [llvm] [CMake] Fix using precompiled headers with ccache (PR #131397)

[David Truby via llvm-commits]

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@@ -446,6 +446,11 @@ if (APPLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
 endif()
 
+# Clang requires this flag in order for precompiled headers to work with ccache.
+if (CMAKE_CXX_COMPILER_ID MATCHES Clang)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -fno-pch-timestamp")
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DavidTruby wrote:

What is the downside to just setting that flag all the time? I assume there must be one or it would be the default. 
I guess occasionally you would have to rebuild the header even if it hasn't changed, or something? I think it's probably OK to just set it unconditionally in that case so that whatever cached build solution people are using continues to work.

https://github.com/llvm/llvm-project/pull/131397